The term "load" is to be understood in the present context in the broadest possible scope. Primarily, it is an electric load, for example an electric motor, a coil, a resistor or the like, and such electric components then may have the "load" proper coupled thereto, for example in the form of an electromagnetic valve, an electric heating or a part to be moved mechanically by an electric motor.
For illustrating the invention in representative manner, an inductive load in the form of an electromagnetic driver device for a braking valve of a brake system is to be considered specifically in the following. Such an electromagnetic driver device is concerned with controlling/adjusting the pressure of brake fluid. The pressure is adjusted by the electromagnetic valve as a function of the current flowing through the coil of the electromagnet. The control signal for such a control/closed-loop control is usually supplied to an electronic switch in the form of a PWM adjusting or control signal (PWM=Pulse Width Modulation), with this PWM control signal opening and closing the electronic switch with a specific pulse duty factor or duty cycle (ratio of pulse duration to pulse interval) so that the current flow through the electronic switch connected in series with the load determines the current flow through the load.
As with other usual closed-loop control circuits, the circuit according to the invention also employs an arrangement in which a first signal source issuing the desired signal as well as a second signal source issuing an actual signal are connected, via a control signal generating circuit, to a controller, for example the control terminal of an electronic switch (for example a MOSFET).
There are numerous possible designs for the control signal generating circuit. Decisive for the output signal of the control signal generating circuit is the deviation between actual signal and desired signal. With slight control deviation, i.e., a little difference between the actual signal and the desired signal, the control signal is of such a nature that only a relatively minor change in the quantity to be controlled or adjusted in the load is still taking place towards the desired signal. With a large control deviation, the control signal is "larger", i.e., there is a faster change of the actual signal for approximation to the desired signal.
In order to accelerate the approximation of the desired signal and the actual signal in case of large control deviations, the control signal is provided with a differential component. The control signal is thus amplified overproportionally in case of large control deviation in order to obtain an as fast as possible approximation of the actual value to the desired value.
The prior art reveals a large variety of measures for obtaining in addition to proportional control also differential and/or integral control, either alternatively or in addition thereto.
In the past few years an increasing number of digital circuits has been employed for controlling/adjusting electric loads. For producing a PWM control signal, a digital control circuit employs, for example, an up/down counter whose continuously changing count is compared to a digital periodical ramp signal. Depending on the ratio of the numerical values compared with each other, the level of the PWM control signal obtained therefrom is either high or low. The up/down counter is clocked by a constant clock signal, with the counting direction being set in accordance with the ratio between desired signal and actual signal.